Currently, telescope users cannot take the full advantage of a fine laser spot to align a telescope at a finest point for achieving higher calibration resolution. This problem is specially evident in a fix-focused laser module installed in a laser collimator for a telescope
A typical collimated laser module installed in a telescope laser collimator is fix-focused at infinity to achieve its maximum projection distance at the smallest divergent angle. In a conventional telescope, the effective focal length (EFL) of the telescope ranges from 1 to 20 feet. At this distance, the collimated laser spot size is almost equivalent to its initial aperture size. Thus, the laser spot appears quite large in this range. This makes it harder for the user to pinpoint the exact optical alignment axis.
Furthermore, the pointing errors of a low cost laser module, typically due to component manufacturing errors, contributes to the focusing problem. One of the problem is from the lens manufacturing process and the lens cap design of the laser module. Almost all lenses have mechanical centration errors due to manufacturing processes. For example, an un-calibrated off-centered grinding machine for glass lens, a none uniform temperature injection flow on a plastic parabolic surface during injection modling of the lens, manufacturing tooling errors causing an off-centered injection mold would contribute to centration errors. In a conventional focusing design for a laser diode module, the collimating lens is installed in a threaded lens cap and the focusing is adjusted by rotating the cap to produce longitude displacement from the laser diode. The lens rotation with the lens cap generates pointing errors from its centering tolerances. In reality, every time when a laser is focused in the conventional method, a laser spot circling is seen about the true center pointing axis on the projecting screen.
A common method for collimating a telescope with laser collimator is by magnifying the laser spot with a barlowed lens where the laser light expands large enough to cover a pre-installed donut shaped sticker in the center of the primary mirror. The return reflection displays a shadow of the black ring within a soft red patch of laser light on the viewing screen of the collimator. Then, the primary mirror is adjusted until the shadow of the black ring is around the exiting aperture of the laser. This method is known as Barlowed Laser Technique.
In another configuration of a collimation apparatus for a Schmidt-Cassegrain Telescope (SCT), the SCT employs a cored primary mirror. A cored primary mirror includes a hole in the center of the primary mirror. This hole renders the methods that rely upon dots, circles or markings at the center of the primary mirror useless. One solution is to set up the telescope point at a distant target outside of the telescope's closest focus distance. Then, align the optics in response to the reflected image by projecting a reticle image on the optics to be aligned, form a reflection of the reticle image from the optics, and display the reflected reticle image on the distance target. Again, the laser spot size is limited to the fixed-focused laser's initial aperature size.
Additionally, geometric alignment of telescope focuser, and optical alignment of the secondary mirror using the laser collimator have been impossible for the users, because the laser collimators do not provide a viewing window to view the projected laser spot on the projecting surface. The conventional laser collimators rely on the returning reflection image from the primary mirror to a viewing screen close to the laser exiting aperature.
In other words, in a telescope collimation using a laser collimator, a user cannot see the projected laser spot on the surface of the aligning optical element due to the location of the installed laser collimator, because the laser collimator is directly in the viewing path. The user typically has to calibrate the secondary mirror by peaking through the opening of the telescope to see the projecting image on the surface of the secondary mirror from the reflected image of primary mirror at a distance.
Therefore, there is need for a method and apparatus to improve the precision of calibration of an optical device, such as a telescope laser collimator.